Organic light-emitting diodes (OLEDs) and organic photocells are structurally similar in that they all have an optoelectronic sub-structure disposed between two electrodes. The optoelectronic sub-structure is capable of converting electrical charge to light or converting photon energy to electrical charge. Typically at least one of the electrodes is made of a transparent material such as Indium Tin Oxide (ITO) so as to allow light to reach the optoelectronic sub-structure for photon to electric charge conversion or to allow light produced in the optoelectronic sub-structure to transmit therefrom.
OLEDs are known in the art. For example, Hung et al. (U.S. Pat. No. 5,776,623) also discloses an electroluminescent device wherein a 15 nm-thick CuPc layer is used as an hole ejecting layer (HIL), a 60 nm-thick NPB layer is used as a hole transporting layer (HTL), a 75 nm-thick Alq3 layer is used as an electron transport layer (ETL). A 0.5 nm-thick lithium fluoride layer is also deposited on the Alq3 layer. The lithium fluoride layer can be replaced by a magnesium fluoride, a calcium fluoride, a lithium oxide or a magnesium oxide layer.
Kido et al. (U.S. Pat. No. 6,013,384) discloses, as shown in FIG. 1a, an organic electroluminescent device 10 wherein the optoelectronic sub-structure consists of a hole transport layer (HTL) 13, a luminescent layer 14 and a metal-doped organic compound layer 15 disposed between an anode layer 12 and a cathode layer 16. The device is fabricated on a substrate 11. According to Kido et al., the organic compounds which can be used in the formation of the luminescent layer, the electron transport layer and the metal-doped layer in the OLED device, include polycyclic compounds, condensed polycyclic hydrocarbon compounds, condensed heterocyclic compounds, etc. The dopant in the metal-doped organic compound layer is a metal having a work function of less than or equal to 4.2 eV. The luminescent layer can be made of Alq3 (an aluminum complex of tris(8-quinolinolato)), for example. The hole transport layer 13 can be made of an arylamine compound. The anode layer 12 is made of ITO and the cathode 16 is an aluminum layer.
Weaver et al. (U.S. Publication No. 2004/0032206 A1) discloses another OLED including an alkali metal compound layer. As shown in FIG. 1b, the OLED 20 is fabricated on a plastic substrate 21 pre-coated with an ITO anode 22. The cathode consists of two layers: a metal oxide layer 28 deposited over a layer 27 of Mg or Mg alloy. The alkali metal compound layer 26 can be made of alkali halides or alkali oxides such as LiF and Li2O. The organic layers include an HTL layer 23, an emissive layer (EML) 24 and an electron transport layer (ETL) 25. In particular, a layer of copper-phthalocyanine (CuPc) is deposited to a thickness of about 10 nm thick over the ITO anode to improve hole injection and device lifetime. A hole transport layer of 4,4′-[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPD) is deposited to a thickness of about 30 nm over the CuPc. An emissive layer of 4,4′-N,N′-dicarbazole-biphenyl (CBP) doped with fac-tris(2-phenylpyridine-)-iridium (Ir(ppy)3) is deposited to a thickness of 30 nm over the NPD. A hole blocking layer of aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolate (BAlq) is deposited to a thickness of about 10 nm over the emissive layer. An electron transport layer of 8-tris-hydroxyquinoline aluminum (Alq3) was deposited to a thickness of about 40 μm over the BAlq. A layer of LiF about 0.5-1 nm thick is deposited after the Alq3 and before the Mg alloy (including Mg:Ag).
Raychaudhuri et al. (U.S. Pat. No. 6,551,725 B2) discloses an OLED 30 wherein a buffer structure is disposed between the organic layer and the cathode. As shown in FIG. 1c, the buffer structure consists of two layers, a first layer 37 containing an alkali halide is provided over the electron transfer layer (ETL) 36, and a second buffer layer 38 containing a metal or metal alloy having a work function between 2.0 and 4.0 eV is provided over the first buffer layer 37. In addition, a hole injection layer (HIL) 33 is provided between the anode 32 and the organic layers. The hole injection layer can be made of a porphorinic or phthalocyanine compound. The hole injection layer can also be made of a fluorinated polymer CFx, where x is 1 or 2. The hole transport layer (HTL) 34 can be made of various classes of aromatic amines. The emissive layer (EML) 35 provides the function of light emission produced as a result of recombination of holes and electrons in the layer. The emissive layer is comprised of a host material doped with one or more fluorescent dyes. According to Raychaudhuri et al., the preferred host materials include the class of 8-quinolinol metal chelate compounds with the chelating metals being Al, Mg, Li and Zn. The cathode layer 39 is made by sputter deposition to provide increased conductivity and reflectivity of the electron ejecting layer of the device.
Photovoltaic devices such as photocells and solar cells are also known in the art. A typical organic solar cell is shown in FIG. 2. As shown in FIG. 2, solar cell 40 consists of a base electrode (ITO) 42 provided on a substrate 41, a buffer layer 43, a ZnPc (zinc-phthalocyanine) layer 44, a fullerene (C60) layer 45, a top buffer layer 46 and a top electrode 47. The base layer can be made of 3,4-polyethylenedioxythiophene:polystyrenesuffonate (PEDOT:PSS), for example. The top buffer layer can be made of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also called bathocuproine or BCP). Together, the ZnPc layer and the fullerene layer make an organic active layer for the solar cell. Thus, a photocell such as an organic solar cell, like an OLED, comprises a top electrode, an organic optoelectronic sub-structure and a base electrode. The basic principle of an organic solar cell can be found in Forrest et al. (U.S. Pat. No. 6,580,027 B2). The commonalities between an OLED and an organic solar cell can be found in Meissner et al. (U.S. Pat. No. 6,559,375 B1).
OLEDs and organic solar cells are organic optoelectronic diodes. Such an optoelectric diode comprises a first electrode and a second electrode separated by one or more active organic optoelectronic layers. In an OLED, electrons and holes are injected from the electrodes through corresponding transport layers into a luminescent layer. The combination of the electrons and holes produces excitons. These excitons produce light in a relaxation process. In an organic solar cell, ambient light produces excitons at the interfaces between the active layer and the adjacent layers. Through a dissociation process, the excitons produce electrons and holes. Through a p-type or an n-type transport layer, the electrons and holes are separately transported to the electrodes, thereby producing electrical currents.
In general, intrinsic carriers do not exist in the organic layers within an OLED or an organic solar cell. In order to reduce the driving voltage in an OLED or to increase the electrical current in an organic solar cell, it is possible to insert a layer of alkali halide or alkali oxide, such as LiF, CsF, Li2O and MgF, between the organic electron transport layer and the metallic cathode (see Hung et al.). The insertion of an alkali halide layer can effectively lower the electron injection energy barrier and thus increase the injection of electrons. However, because alkali halides are good insulation materials, the inserted layer must be sufficiently thin in order to produce a tunneling effect. Alternatively, n-type dopants, such as Cs, Li and Mg that have strong electron-donating characteristics can be incorporated into the organic election transport layer by way of co-deposition (see Kido et al.) As such, the Fermi energy level of the organic electron transport layer can be brought closer to the lowest unoccupied molecular orbital (LUMO) energy level (see Forrest et al. regarding LUMO in a photocell structure). However, because these types of dopant metals are chemically active, they may not be suitable for use in the thermal evaporation process that is commonly used in OLED manufacturing.
It is thus advantageous and desirable to provide a method and a device structure to increase the operations efficiency in an optoelectronic device such as an organic light emitting diode or an organic photocell.